COMPLETE PAKASITISM OF TEICHINA. 



103 



exists in various degrees, and, as a rule, attains its complete develop- 

 ment only at the expense of a free life. 



The most complete case of this parasitism has not, however, 

 hitherto found a place in our exposition. I refer to Trichina, which, 

 as a rule, completes its entire life-history in the body of its host. 

 The embryos, which ' are born alive, soon bore through the wall 



Fig. 66. — Trichina spiralis. A. Kmbryo ; B. 

 Intermediate form ; C. Sexual form ; 

 (unimpregnated female). 



of the intestine which shelters their parents, and thus reach the 

 muscles, where they develop into a larval form, which, after trans- 

 ference into another suitable host, directly completes its growth into 

 the sexual form (Fig. 66). A lengthened existence in the free state is 

 thus entirely excluded ; even embryonic development and migration 



occur during the period of parasitic life. It is exceptional, and only 

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